Xerographic development method and apparatus



XEROGRAPHIC DEVELOPMENT METHOD AND APPARATUS Filed April 30, 1965 Oct. 14, 1969 MAYER ETAL 3 Sheets-Sheet INVESTORS W CCFEM A $055M J r,

BY 2r 2 .4 T TORNE VS E. F. MAYER ET AL XEROGRAPHIC DEVELOPMENT METHOD AND APPARATUS Filed April 30, 1965 3 Sheets-Sheet 2 INVENTORS EDWARD F. MAYER WILLIAM A. SULLIVAN, M

A T TORNE Y5 XEROGRAPHIC DEVELOPMENT METHOD AND APPARATUS Filed April 30, 1965 0d:- 14, 1969 MAYER ETAL 3 Sheets-Sheet 3 INVENTORS RD F. MAYER IAM A. SULLIVAN,Jr.

A T TORNEYS United States Patent 3,472,657 XEROGRAPHIC DEVELOPMENT METHOD AND APPARATUS Edward F. Mayer, Pittsford, and William A. Sullivan, Jr.,

Rochester, N.Y., assignors to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Apr. 30, 1965. Ser. No. 452,098

Int. Cl. G03g 13/08, 15/08 U.S. Cl. 96-1 16 Claims ABSTRACT OF THE DISCLOSURE A toner deposit is formed by forming an electrostatic charge on a surface and thereafter flowing spaced apart from the surface but within the effective field of the electrostatic charge a two-component developer mix comprising electroscopic developer particles carried by granular carrier particles.

This invention relates to xcrography and particularly to improved method and apparatus for development of electrostatic charge patterns.

In the process of xerography, for example, as disclosed in Carlson patent, US. 2,297,691, a xerographic plate comprising a layer of photoconductive insulating material on a conductive backing is given a uniform electric charge over its surface and is then exposed to the subject matter to be reproduced, usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the radiation intensity that reaches them, and thereby creates an electrostatic latent image on or in the photoconductive layer. Development of the latent image is effected with an electrostatically charged, finely divided material such as an electroscopic powder that is brought into surface contact with the photoconductive layer and is held thereon electrostatically in a pattern corresponding to the electrostatic latent image. Thereafter, the developed xerographic powder image is usually transferred to a support surface to which it may be fixed by any suitable means.

Various developing systems are well known and include cascade, brush development, magnetic brush, powder cloud, and liquid development to name a few. Each of these are known in the art to have special attributes rendering them most suitably appropriate for particular development applications. Of those named, the cascade system enjoys by far the greatest commercial utility and employs a two-component developer composition as for example disclosed in patents, US. 2,618,551 and 2,618,- 552. The two components include a relatively hard and large particle termed carrier which serves to transport and trib-oelectrically charge the relatively smaller electroscopic marking particles termed toner. As the developer is caused to cascade over an image bearing surface, the toner is attracted away from its carrier to the charge pattern and effect its development.

Inherent in the cascade development system is the triboelectric relationship which is fostered between the carrier beads and the toner particles which generally become charged polar opposite to that of the charge pattern to be developed thereby. As the two component mixture passes over a charged area, it is characteristic that the force of the charge pattern exceeds that of the carrier to denude the latter permitting the toner to cling to the pattern and render it visible. This has proved a most effective and economical process producing excellent high quality, high fidelity reproductions particularly where the pattern is comprised principally of line copy as opposed to solid area, half-tone, continuous tone or the like. Where the pattern being reproduced contains at least partial areas lieved attributed to inherent competing properties of the cascade system and which reach a state of equilibrium in the course of effecting image development. The first of the properties is that of deposition in which the toner is attracted, as intended, to the superior electrostatic field emanating from the image charge pattern. At the same time, unwanted spurious toner deposit occurs in a statistical pattern as a result of toner insufficiently charged to be retained by their carrier. The latter deposits randomly about the surface of the xerographic plate relatively independent of the plate fields as by Van der Waals forces. Still other deposits occur by the weakly held toner being attracted to the residual plate charges corresponding to the non-image or background areas of the pattern being reproduced.

Opposing toner retention after deposition, even in the image areas, is the inherent property of carrier scavenging. This is attributed to acquired charges associated with denuded carrier particles having previously deposited toner onto an image pattern. As the denuded carrier particles pass downstream, their continued frictional contact causes an inherent buildup of triboelectric surface charge which competes with the image charge patterns for attraction of previously deposited toner particles. Whereas the scavenging effect is beneficial in removing loosely held unwanted toner deposits in the background non-image areas, it is likewise detrimental to the extent that central portions of solid area images having weak electrostatic fields become depleted and appear developed only about the fringe perimeters of the image area.

As should be appreciated, these competing factors have imposed carefully defined restrictions on the relative ratio of toner to carrier in the developer mix. Excessive, insufficiently charged toner increases the undesirable background deposition with reduced scavenging while reduced toner concentrations result in insufficient image densities caused by excessive scavenging. In the case of line copy images it has been possible to obtain an acceptable balance of these factors by maintaining toner to carrier concentrations closely controlled at about 1%. percent by weight. It has not been possible previously, however, to similarly obtain a satisfactory balance for image patterns including other than line copy and specifically solid area characterized by wide differences in charge density as between the central and perimeter portions thereof and continuous tone consisting of wide variations in charge gradients. These problems have likewise rendered the cascade system unsuitable for sequential developments in forming a composite as from separation originals in a plurality of colors without intermediate fusing or transfer even for line copy reproductions. Because of the scavenging effect, subsequent developments deplete previously developed images.

Many only partially successful endeavors have been tried to overcome this basic handicap of an otherwise highly suitable and highly utilized development system. Among the approaches has been the use of an electrode in the development zone spaced opposite the image area. The electrode, which improves the magnitude of the field strength in the central portions of solid areas, is known to alleviate but not eliminate the problem.

Now in accordance with the instant invention, there has been discovered a novel development system employing cascade developer mix able to effect high quality development of image patterns without regard to the type of image formation represented thereby and with substantially increased latitude of mixture concentrations. Specifically, it is possible with the system of the invention to develop line copy, solid area, half tone, continuous tone and even sequential color development as aforesaid with high fidelity and without the imposed limitations of mixture concentrations previously existent in the art. This is achieved in accordance with the method and apparatus of the invention by cascading high concentrations of developer mix physically removed from the image surface on an electrode support, but carefully maintained within the electric field of the charge pattern. Toner passing the charge pattern is attracted selectively thereto in effecting development which is unaffected by subsequent scavenging. The techniques hereof have been found to produce such insignificant amounts of background deposition as to render scavenging unnecessary for the removal thereof.

It is accordingly an object of this invention to provide an improved system for development of xerographic images having greater development latitude than heretofore.

It is a further object of the invention to provide an improved cascade development system for developing charge patterns with controlled or an absence of carrier scavenging.

It is a still further object of the invention to provide an improved two component cascade development system having high quality solid area and continuous tone development capability in developing xerographic charge patterns.

These and other objects of the invention are achieved in accordance with the embodiments exemplified by the following drawings in which:

FIG. 1 schematically illustrates an automatic drum type xerographic apparatus employing the developing system hereof;

FIG. 2 schematically illustrates an automatic flexible plate xerographic apparatus employing the development system hereof;

FIG. 3 is a schematic variation of FIG. 2 as adapted for two color development;

FIG. 4 is a side elevation of a manually operable apparatus for developing fiat xerographic plates in accordance herewith;

FIG. 5 is a schematic apparatus for an out-of-contact belt system loaded in accordance herewith; and

FIG. 6 is a schematic apparatus for a touchdown development loaded with developer in accordance herewith.

For a general understanding of the xerographic processing system in which the invention is incorporated, reference is had to FIG. 1 in which the various components are schematically illustrated. As in all xerographic systems based on the concept disclosed in the above-cited Carlson patent, a radiation light image of copy to be reproduced is projected onto the sensitized surface of a xerographic plate to form an electrostatic latent image thereon. Thereafter,-the latent image is usually developed with an oppositely charged developing material to form a xerographic powder image, corresponding to the latent image on the plate surface. The powder image is then electrostatically transferred to a support surface to which it may be fused by any suitable form of fusing device whereby the powder image is caused permanently to adhere to the support surface.

The xerographic apparatus described herein typically may be of a type disclosed in patent, US. 3,076,392, in which copy to be reproduced is placed on a support tray 10 from which it is fed onto a transport mechanism generally designated as 11. Suitable drive means are provided for the transport mechanism from motor 12 to endless belts 13 whereby the copy is moved past the optical axis of projection lens system 14 that is illuminated by a projection lamp LMP-l. The image of the copy is reflected by a mirror 15 through an adjustable objective lens 16 and then reflected by a mirror 17 downwardly through a variable slit aperture assembly 18 and onto the surface of a xerographic plate in the form of drum 19.

Cir

Xerographic drum 19 includes a cylindrical member mounted in suitable hearings in the frame of the machine and is driven in a clockwise direction by a motor 24 at a constant rate that is proportional to the transport rate of the copy, whereby the peripheral rate of the drum surface is identical to the rate of movement of the reflected light image. The drum surface comprises a layer of photocon ductive material on a conductive backing that is sensitized to a potential of about 300l,000 volts prior to exposure by means of a corona generating device 25, which may be an adaptation of the type disclosed in patent, US. 2,836,- 725, and which is energized from a suitable high potential source. Optionally and preferably the photoconductive surface is overcoated with an adhesive transparent material, such as Teflon, which facilitates transfer of the developed image and eases subsequent cleaning of the surface before recycling.

The exposure of the drum to the light image discharges the photoconductive layer in the areas struck by light, whereby there remains on the drum a latent electrostatic image in image configuration corresponding to the light image projected from the copy, As the drum surface continues its movement, the electrostatic latent image passes through a developing station 26 at which a two-component developing material 27 of a type described above and as will be further described below is cascaded juxtaposed and out of contact with the plate surface to effect development of the image thereon. Thereafter, the powder image passes through an image transfer station 42 at which the powder image is electrostatically transferred to a support surface web 43 by means of a second corona generating device 44 similar to corona charging device 25 mentioned above.

The support surface to which the powder image is transferred may be of any convenient type, such as paper, and is obtained from a supply roll 45 and is fed over guide rolls 46 and 47 and over suitable tensioning rolls being directed into surface contact with the drum in the immediate vicinity of transfer corona generating device 44. After transfer, the support surface is separated from the drum surface and guided through a suitable fusing apparatus 48 whereby the powder image is permanently aflixed thereto. Thereafter, the support surface is fed over a further system of guide and tensioning rolls and onto a take up roll 52 that is driven by motor 53.

After transfer, the xerographic drum surface passes through a cleaning station 55 at which the surface is brushed by a cleaning brush assembly 56 rotated by a motor 57 whereby residual developing material remaining on the drum is removed. Thereafter, the drum surface passes through a discharge station 58 at which it is illuminated by a fluorescent lamp LMP2 whereby the drum surface in this region is completely flooded with light to remove any electrostatic charge that may remain thereon. Suitable light traps are provided in the system to prevent any light rays from reaching the drum surface, other than a projected image, during the period of drum travel immediately prior to sensitization by corona generating device 25 until after the drum surface is completely passed through the developing station 26.

Development in accordance with the invention hereof is effected by means of an apparatus 60 comprising a housing 61 and including a continuously operative offset conveyor 62 being driven by a motor 63. The two-component developer material 27 is contained in the sump of the housing and is carried up by the conveyor for release onto an inclined chute 64. On the chute, the developer cascades downwardly in a direction towards the surface of drum 19 until striking the forward upper dependent wall 65 of the housing. The wall interception deflects the developer flow in order to effect its cascade onto a support member 66 extending from about a four oclock drum position generally radially parallel and curvilinear to the circumference of the rotating drum 19. Spacing of support 66 to the drum surface for developer compositions as will be described is on the order of approximately 0.03 to 0.12 inch with ranges of approximately .07 to .09 inch preferred. This spacing has been determined to be optimum for conditions defined below in permitting the developer to remain out-of-contact with the drum surface while still maintaining the developer within the effective fields of the image charge pattern. Toner component 28 of the developer consumed in developing is stored in dispenser 58 and is controllably released by means of gate 59.

Support 66 can comprise any suitable material having sufficient durability and rigidity for uniformly maintaining the close spacing tolerances to the drum periphery during continuous operation and including electrically insulating as well as electrically conductive materials. Preferably, support 66 comprises an electrode member which is effective to enhance the development field of the image pattern as is associated with solid areas in order to obtain maximum benefits from the development techniques hereof. Preferably also, the electrode support member is electrically biased relative to drum 19 by means of a potential source 67 from about the magnitude of the background potential on the drum surface up to about 200 volts. With a rotating drum of the type illustrated in FIG. 1, approximately three inches of curved electrode is about adequate for peripheral drum speeds of up to about 6 inches per second. For higher drum speeds an opposite hand developing apparatus 68 as shown dashed can be employed to extend the development zone to about the eight oclock position.

The developer employed can comprise basic carrier beads of about 2.2 specific gravity and toner compositions both known in the art as for example, described above and as further described in patent, U.S. Reissue 25,136. The carrier beads for use herein, in contrast to about a 750 micron dimension in the prior art, preferably has a size range of approximately 100-300 microns generally about 250 microns and spherical as to provide substantially increased surface area of the carrier particles. With this size carrier, the toner concentration is preferably about 6-10 percent by weight usually about 8, as opposed to approximately 1% percent by weight in the prior art. The higher toner concentration coupled with the small carrier dimension and higher density provides increased carrier surface area and a low average charge per toner particle to be easily attracted to the image charges. By employing carrier beads of about 4-5 specific gravity, improved mechanical flow action can be realized. The latter will, of course, reflect a lower concentration of toner by weight to generally about 1 to 5% with about /2 to 2% deemed optimum. Since the carrier beads are maintained separated from the image drum surface, scavenging is non-existent and deposited toner is retained by the electrostatic image forces Which need only be adequate to attract the powder without regard to retaining it in subsequent competition with the carrier as was inherent in the prior art. This is particularly true as associated with the central image portions of solid area.

Having described the basic concept of the invention refer now to FIG. 2 in which there is shown a variation of the invention hereof as further adapted for high speed operation. Principally, the apparatus embodiment hereof differs from that of the previous embodiment in that the xerographic plate comprises an endless flexible belt 70 such as vitreous selenium on a conductive flexible substrate. The belt is continuously advanced over guide rolls by means of a motor 71 and is uniformly charged in the course of its movement by means of corona generator 25. A copy sheet is supported at plane 72 and is exposed to the charged belt surface by means of objective lens 73 upon being illuminated by means of a pair of flash lamps 74 and 75. Development is then effected as the belt proceeds past developing apparatus 78 which is similar to apparatus 60 above, but having a straight developer support 66 extending parallel to the moving path of plate 70 at about 20 to 90 degrees to the horizontal and usually about 30 to 70 degrees. The support provides extended electrode surface for extending the eifective development zone. By an appropriate electrode length in either a single or duplex unit as shown, the apparatus can be adapted to accommodate belt speeds up to approximately 40 inches per second and above. A desired electrode length is to a large degree a function of belt speed, developer flow as well as toner concentration. The duplex unit shown is highly beneficial in minimizing directional effects from the developer flow.

The developed image is then transferred from the belt as it passes over guide roll 76 to transfer web 43 by means of corona generator 44 similarly as above. After transfer, the surface of the belt is cleaned before being recycled via a soft cotton web 79 passing counterdirectional to the belt over guide 82 in contact therewith from a supply roll 80 to a takeup roll 81 being driven by motor 83.

As shown in FIG. 3, the development system of the invention is adapted for a two-color reproduction. As employed herein, the belt 70 is first charged by corona generator 25 and then exposed to an original such as a separation original supported at exposure plane 72. The image charge is then developed in a first color by a developing unit 111 containing a two-component developer with toner of the appropriate reproduction color.

With a first developed image on its surface belt 70 is recharged by a second corona generator 112 and then reexposed through lens 114 to a second original supported at exposure plane 113. The charge pattern formed by the second exposure is then developed in developing unit 115 containing a two-component developer with toner of the appropriate reproduction color differing from that in unit 111. Thereafter the belt is processed similarly as described in FIG. '2 above to result in transfer of a two-color reproduction onto support 43. Suitable mechanisms or electric circuitry (not shown) can be incorporated to ensure optical registration between the different exposures onto belt 70 as to result in a single composite reproduction on support 43.

In FIG. 4, there is illustrated apparatus adapted to carry out the development method hereof particularly suitable for flat xerographic plates which have been remotely processed for forming the image charge pattern. The apparatus is comprised of a horizontal tray 87 having opposite end pockets 88 and 89 formed therein for containing developer during its inactivity and to contain the developer passing from the opposite pocket during operation. On the inside terminal lips of the pockets there is attached spring clasps 90 and 91 by which a Xerographic plate 86 is received and secured image side down uniformly spaced and parallel to bottom tray wall 92. On the underside of bottom wall 92 there is secured a downward depending boss 93 axially drilled and mounted on pin 94 in stand 95.

Development is effected by oscillating or rocking tray 87 about the pivot such that developer composition 27 is caused to cascade from one pocket to the other and pass on the bottom wall in the vicinity of the image pattern on plate 86. End stops 96 and 97 each include adjustable posts 98 that are secured to limit the oscillatory travel of the tray in either direction. 20 to 90 degrees is regarded as about the limiting range of angular displacement for cascading the developer out of contact with the plate surface. About 3070 degrees is usually adequate and optimum and about 2 to 4 passes of developer will produce the high quality development expected of the technique hereof.

In FIG. 5 there is illustrated an apparatus wherein the development system hereof, rather than for direct image development, is employed for loading the surface of an endless donor belt that is continuously advanced by means of a motor 106. The belt surface receives a uniform application of electrostatic charge from a corona generator 107 before passing vertically inclined downwardly and in closely spaced relationship to the cascading developer material which deposits a uniform layer of toner material thereon. On emerging from the development mechanism, the belt surface, containing a uniform coating of toner particles on its exterior surface passes upwardly approaching a tangential spacing with the pattern bearing surface of rotating drum 19. Spacing between belt 105 and the drum is maintained by means of guide rolls 109 and 110 as not to exceed between approximately .003 to .05 inch at the closest point. Secured behind the belt at the point closest to the drum surface is a vibrator 108 which can be actuated mechanically, electromechanically, piezoelectrically or acoustically. The vibrator effects an oscillatory motion on the belt surface to dislodge toner thereon and produce a reduction of mechanical Van der Waal forces permitting the image field to more easily attract the toner across the gap to the image pattern on the drum surface and effect development thereof. Potential source 116 maintains a positive bias on xerographic drum 19 for suppression of background potential as before.

In FIGURE 6 belt 105 is operatively advanced in continuous contact with the surface of drum 19 in a manner disclosed in Mayo patent, US. 2,895,847. Uniform load ing of belt 105 is achieved in accordance with invention hereof by developer unit 78 which is effective for rapidly loading the belt surface with a uniform coating of toner particles.

By the above description, there is disclosed novel methd and apparatus for selectively applying electroscopic powders to charge patterns by cascade techniques employing a two-component developer material which cascades out of contact with the charge bearing surface. Where the pattern is formed xerographically and includes solid areas, half tone or the like, development is effected with a greater order of fidelity than heretofore. Not only is development hereof highly suitable to monocolor development, but the techniques hereof can be readily adapted to multicolor formation to achieve a result unable to be achieved by cascade techniques of the prior art. As described above, this latter result can readily be accomplished in accordance with the invention hereof by s quentially developing sequential exposures with developers of different colors and without the necessity of intermediate transfer or fusing of previously developed images. Since plate scavenging is absent from the development system hereof previous developments on the plate will remain unaffected during subsequent development cycles.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that this description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A process for applying electroscopic powder particles onto an insulating support comprising the steps of:

(a) producing an electrostatic charge on the surface of an electrically insulating support on which the electroscopic powder is to be applied; and

(b) flowing spaced apart from said support within the effective field of the electrostatic charge thereon a quantity of two-component developer mix comprising electroscopic developer particles on the surface of granular carrier particles.

2. The process according to claim 1 in which saidcharge comprises an image pattern to be reproduced.

3. The process according to claim 2 in which said electrically insulating layer comprises a photoconductive insulator.

4. The process according to claim 1 in which the flow of said developer is maintained uniformly within the field of said charge by means of an electrode support.

5. The process according to claim 4 in which the sup porting surface of said electrode support is positioned subjacent and substantially parallel to said insulating support.

6. A process for applying electroscopic powder particles in at least two colors onto an insulating support comprising the steps of:

(a) producing a first electrostatic charge pattern on the surface of an electrically insulating support on which a first color electroscopic powder is to be applied;

(b) flowing spaced apart from said support within the effective field of the electrostatic charge pattern thereon a quantity of two-component developer mix comprising electroscopic developer particles of a first color on granular carrier particles to permit said developer particles to be attracted to said charge pattern;

(c) while the particles of said first applied developer are loosely held by said first charge pattern, producing a second charge pattern on said support on which a second color electroscopic powder is to be applied; and

(d) flowing spaced apart from said support within the effective field of the electrostatic charge pattern thereon a quantity of two-component developer mix comprising electroscopic particles of a second color on granular carrier particles to permit said second color developer particles to be attracted to said second charge pattern.

7. Apparatus for depositing electroscopic developer particles on an electrostatic charge pattern comprising in combination:

(a) a first support means to support an electrically insulating member having an electrostatic charge pattern-bearing surface inclined from the horizontal;

(b) a second support means to support a quantity of two component developer mix including electroscopic developer particles on granular carrier particles, said second support means being arranged extending uniformly opposite said surface of said insulating member and spaced apart therefrom a predetermined distance to support said developer mix out of contact with said surface within the effective field of said charge pattern thereon; and

(c) means arranged to effect a gravity cascade flow of said developer mix on said second support whereby electroscopic developer particles of said developer mix are attracted to said charge pattern on said surface of said insulating member.

8. Apparatus according to claim 7 irr which said second support comprises an electrode.

9. Apparatus according to claim 8 in which the supporting surface of said electrode is positioned subjacent and substantially parallel to said insulating member.

10. Apparatus according to claim 7 in which said member comprises a photoconductive insulator.

11. Apparatus according to claim 10 in which the charge pattern comprises an image pattern to be reproduced.

12. Apparatus for uniformly depositing electroscopic developer particles on a substantially uniformly charged surface comprising in combination:

(a) means to support an electrically insulating member having a substantially uniform electrostatic charge applied to a surface thereof;

(b) second support means to support a quantity of two component developer mix including electroscopic developer particles on granular carrier particles, said second support means being arranged uniformly opposite said surface of said member and spaced apart therefrom a predetermined distance to support said developer mix out of contact with said surface and within the effective field of said uniform electrostatic charge thereon;

(c) means arranged to flow said developer mix on said second support whereby electroscopic developer particles of said mix are attracted to said uniform electrostatic charge on said surface; and

9 10 (d) means operable after depositing said electroscopic from the horizontal and said means to effect flow developer particles on said member for presenting including means to effect a gravity cascade of said said member into developing relation with an image developer mix on said second support. charge pattern on a third support. 15. The apparatus according to claim 14 in which the 13. Apparatus according to claim 12 in which said xerographic plate comprises a drum mounted for rotation. third support comprises a photoconductive insulator layer 16. The apparatus according to claim 14 in which said carried on the surface of a drum, rotatable about an axis, support comprises an electrode. said drum being mounted adjacent said surface of said member and said axis being arranged substantially parale c s Cited lelltf sIaid surface 01f1 said menber. t 1 d UNITED STATES PATENTS n a xerograp 1c repro uc ion appara us 1116 u mg a xerographic plate and means to form an image charge g pattern on a surface of said plate of an original to be 291O963 11/1959 reproduced, a developing apparatus for developing said 2927554 3/1960 i 96 1 X charge pattern comprising in combination: 3O60020 10/1962 Gr i 00m 12 (a) container means to contain a supply of two-com- 3084043 4/1963 G '1 ponent developer mix including electroscopic de- 3146688 9/1964 ac ark et a1. 1l8637 X veloper particles and granular carr er partlcles, 3 232 190 1/1966 Wmmott 951 7 (b) support means extending opposite said surface of 3,257,223 6/1966 King X said plate and substantially parallel to said surface to support a quantity of said developer mix out of GEORGE E LESMES, Primary Examiner contact with said surface and Within the effective field of said image pattern thereon; and VAN HORN Assistant Exammer (0) means to effect flow of a quantity of developer U S Cl XR mix from said container means on said support, said surface and said support being arranged inclined 117-175; 118-637; 355--3 

